Indium tin oxide (referred to as ITO) has been known as a heat-ray shielding material. There has hitherto been known, as a method for producing an ITO powder, a method in which an indium-containing aqueous solution is mixed with a tin-containing aqueous solution to co-precipitate indium tin hydroxide, and firing the obtained co-precipitate.
For example, Japanese Unexamined Patent Application, First Publication No. Hei 6-295315 (Patent Literature 1) describes a method for producing an indium tin hydroxide in which indium metal is dissolved in nitric acid and a mixed aqueous solution with an aqueous tin tetrachloride solution (SnCl4) is concentrated while heating. Japanese Patent No. 2,715,859 (Patent Literature 2) describes a production method in which an aqueous indium trichloride solution is mixed with an aqueous tin tetrachloride solution and the obtained mixed solution is added dropwise in an aqueous ammonium hydrogen carbonate solution.
According to the production method of Patent Literature 1, white indium tin hydroxide is obtained. According to the production method of Patent Literature 2, the formed indium tin hydroxide also exhibits a white color. Furthermore, it has been known that indium hydroxide generally exhibits white color and tin hydroxide also exhibits white color (Non Patent Literature 1, etc.).
It has been known that, in case of using an ITO powder as a heat-ray shielding material, the powder preferably has a color tone in which a<0 and b<0, and the ratio [(a·b)/L] of the (a·b) value to the L value is 0.3 or more in the Lab colorimetric system, so as to enhance heat-ray shielding properties (Japanese Unexamined Patent Application, First Publication No. 2007-154152: Patent Literature 3). However, a conventional ITO powder is inferior in heat-ray shielding properties since the above (a·b)/L ratio has a small value.
Japanese Patent Application No. 2005-232399 (Patent Literature 4) discloses an ITO powder whose powder has a color in which L=52 to 90, a=−5 to −0.1, and b=−4 to 30 in the Lab colorimetric system. However, the ITO powder described in Examples thereof has not a yellow-based tone, but has a dull color tone in which a=−5.8 to −4.6 and b=−12 to 4.6. Japanese Patent Application No. 2003-215328 (Patent Literature 5) discloses an ITO powder in which L=52 to 80, a=−10 to −0.1, and b=−14 to 20. However, the ITO powder does not have a yellow-based tone and any production method thereof is not described at all.
Japanese Patent Application No. 2005-322626 (Patent Literature 6) describes an ITO powder whose powder has a color in which L=82 to 91, a=−8 to 2, and b=0 to 10 in the Lab colorimetric system. However, the ITO powder is a white-based powder in which a white needle-shaped indium obtained by concentrating an indium nitrate solution is calcined and pores thereof are impregnated with tin tetrachloride, followed by firing, and also the production method is complicated.